Hmm, I don't even know what "white nationalist" mean. Nationalism is often confused with chauvinism. The difference is rather important. Nationalism is a natural and mostly positive attitude: a person lives at some place (country, city, village), surrounded by people with that person spends most of the time, with whom that person talks, trades. So it is natural to be more eager to help those people around than someone living on the other side of the globe.
On the other hand, chauvinism is a negative attitude, this is misunderstood nationalism. The good example is the following.
Let's say I have kids and my neighbour too. Obviously I want the best for my kids, so I take care of them, send them to a good school, so they have the best possible education and perspectives - this is "nationalism". It does not mean that I wish anything bad for my neighbour and his kids.
Now, chauvinism means that I am going to hurt my neighbour's kids, since they are competition for mine, and this is obviously bad.
So, coming back to the FB declaration, "white nationalist" does not make much sense. The colour of the skin is hardly a predictor of common interests. I assume FB means chauvinism, otherwise they will have to ban all international sport competitions news (funs are naturally nationalistic since they support national team), they will have to also ban all national holidays, including 4th July, as they are nationalistic by very nature.